1. Field of the Invention
The present invention relates to phase shifters and more particularly to a compact and continuously variable phase shifter based on solid state semiconductors.
2. Description of the Related Art
Current commercial variable phase shifters, like for example those sold from TRAK Microwave Corp. (Florida), typically comprise varactor diodes which have limited maximum phase shifts of less than 120xc2x0 and limited bandpass operation of less than 1 GHz.
However, changing the operation bandpass requires changing the appropriate diode coupling impedance elements. Moreover, phase shifts of more than 120xc2x0 can only be obtained by cascading more than one device.
Another type of prior art variable phase shifter comprises a coplanar-waveguide transmission line periodically loaded with MEMs variable capacitors, i.e. capacitors realized through arrays of microelectromechanical switches. Analog control is provided by electrostatic control of the capacitors. An introductory note to MEMS switches can for example be found in xe2x80x9cRF-MEMS Switches for Reconfigurable Integrated Circuitsxe2x80x9d, Elliott R. Brown, IEEE Transactions on Microwave Theory and Techniques, Vol. 46, No. 11, November 1998. A coplanar-waveguide transmission line is disclosed in xe2x80x9cDistributed MEMS True-Time Delay Phase Shifters and Wide-Band Switchesxe2x80x9d, N. Scott Barker, IEEE Transactions on Microwave Theory and Techniques, Vol. 46, No. 11, November 1998.
The disadvantage of this second approach is that a large number of these capacitors are needed to have a significant combined effect on the loaded transmission line for phase control due to limited (about 30%) variation in the MEMs capacitance values. For example, a 10 mm loaded line with 32 MEMs air bridges is required to provide a 2xc2x0/GHz phase shifting.
Still another type of variable RF phase shifter comprises a voltage variable optical coupler fabricated from LiNbO3, combined with an optical fiber delay line. See for example xe2x80x9cA Novel Wide-Band Tunable RF Phase Shifter Using a Variable Optical Directional Couplerxe2x80x9d, K. Ghorbani, A. Mitchell, R. B. Waterhouse, and M. W. Austin, IEEE Transactions on Microwave Theory and Techniques, Vol. 47, No. 5, May 1999.
Again, the maximum phase shift of this device is fairly limited, with a tunable range from 40xc2x0 to 80xc2x0 at a maximum operating frequency of only 80 MHz. Also, the insertion loss of this device can be up to 8 dB at 80 MHz. Furthermore, devices based on LiNbO3 are quite expensive.
There is, therefore, a need for a simple, compact-size and low cost phase shifter which is able to obtain broadband, continuous 360xc2x0 phase shifting also at Gigahertz frequencies.
The present invention relates to a novel, compact size, continuously variable phase shifter, in particular for use in the radiofrequency (RF) domain. This novel phase shifter can also be applied to other bands lower in frequency such as VHF (Very High Frequency), UHF (Ultra High Frequency), or higher frequency bands such as microwave or millimeter wave as well. According to a first aspect of the present invention, a phase shifter is disclosed, which comprises a digital binary microelectromechanical (MEMs)-based phase shifter for coarse phase tuning, and a resistor, inductor and capacitor (RLC) network for fine phase tuning serially connected to the digital binary coarse phase shifter, wherein fine phase tuning is achieved by continuously varying the resistance value through variable optical power.
According to a second aspect of the present invention, a continuous phase shifting method is provided, comprising the steps of: providing a RLC network having a resistor, an inductor and a first capacitor; and optically continuously varying the resistance value of the resistor.
As a result, a very compact size variable phase shifter is obtained with broadband, continuously variable phase shifting from 0xc2x0 to 360xc2x0. This phase shifter can be fully integrated on conventional semiconductors such as Si or GaAs. According to a preferred embodiment of the present invention, the RLC network acts as a low pass filter with a very low insertion loss and a close to linear phase delay versus frequency response. Moreover, the phase shifter according to the present invention has the capability of broadband operation. Its bandwidth can be easily varied by changing the values of the inductor L and/or capacitor C in the RLC network. Thus, by switching the values of these parameters using another set of MEMs switches, a compact phase shifter with the capability of in-situ bandwidth switching can be realized.
Therefore, the advantages of the present invention can be summarized as follows: compact size, broadband operation with an in-situ switchable bandwidth, close to linear phase shift versus frequency resulting in true time delay capability, very low insertion loss and high value of maximum phase adjust.
Further to this, it has to be noted that in the phase shifter according to the present invention less than 10 MEMs switches combined with the RLC network are required in order to obtain continuous 360xc2x0 phase shifting. Moreover, the maximum phase adjust is greater than in prior art devices.
Still a further advantage of the present invention is that of providing an inexpensive device, due to the fact that commercial Si or GaAs technologies are used.